1. Field of the Invention
This invention relates to a method and apparatus for calibrating relatively large grids, such as reseau grids and more particularly relates to a method for calibrating large grids using a calibrated grid in a comparison test.
2. Description of the Prior Art
The use of grids, such as a reseau grid, for referencing measurements on large format photography is a well known and widely practiced technique. However, the difficulty in accurately and reliably calibrating large grids on the order of 9 inches by 126 inches has been a basic problem. Calibration of grids, such as reseau grids, involves more than nominally measuring the arbitrary (X,Y) location of a set of randomly located intersections. Rather, the measuring task is the determination of the small distances, often on the order of a few microns, the actual intersections or points are from the desired perfect pattern location.
Presently employed grid calibrations for standard size grids are usually made with a comparator using, for example, a four-rotation measurement technique to reduce systematic measurement errors. However, even where some of the larger comparators can be used with a grid pattern having dimensions as large as 10 inches by 36 inches, the four-rotation measurement technique cannot be used since the grid can only be rotated for direct and reverse readings. The calibration of even larger grid patterns by using comparators is not possible. Other calibration techniques employ the measurement of absolute distances of the intersection points in the X and Y directions and comparing the absolute measurements with the desired distances. However, as grid size increases to several feet in one dimension, all known and presently employed grid calibration methods are unable to provide acceptable accuracy without elaborate equipment in carefully controlled laboratory conditions.